Kaolin, a naturally occurring form of hydrated aluminum oxide commonly known as hydrated clay, is widely used in paper coatings, as a filler in paper manufacture, as a paint additive, and in coatings for wire and cable. For many applications, the kaolin clay must first be calcined in order to take on certain desirable characteristics. Raw kaolin or aqueous kaolin slurries are calcined, i.e. brought to an elevated temperature, such that the clay loses its water of hydration and becomes a powdered solid. It is the calcined kaolin which has the desirable properties of increasing the whiteness and raising the level of opacity of materials coated with it.
Calcined kaolin clay retains its desirable properties even after it is reslurried. Since calcined kaolin is used in large quantities as an aqueous slurry, it is desirable to be able to ship it as a high solids aqueous slurry. Unfortunately, these slurries are fairly unstable.
Calcined clay particles have a highly irregular shape which is responsible for their property of increasing opacity. However, this irregular shape is also a major cause of calcined clay slurry instability. As a calcined clay slurry is agitated, the irregularly shaped particles sweep out an abnormally large volume, thereby increasing the statistical likelihood of particles colliding, forming agglomerates and settling out. This is known as dilatant settling. Dilatant settling and slurry instability result in the formation of hard-pack solids precipitation. This hard-pack renders the slurry useless as an article of commerce and potentially damaging to piping, pumps, and other slurry handling mechanical equipment.
The large volume swept out by the particles also dramatically increases the high shear viscosity of calcined clay slurries. This high shear viscosity, characteristic of calcined clay slurries, presents major handling problems for the industry because the calcined clay slurries are applied to substrates under high shear conditions. In order to prevent the effects of dilatancy on slurry viscosity and slurry stability, the level of solids of a calcined kaolin clay slurry must be kept low (40 to 50% by weight) relative to non-calcined clay slurries which can have levels of clay solids of about 70% by weight.
These problems have been the subject of extensive research and have been long standing problems for the calcined clay industry. U.S. Pat. No. 4,017,324 teaches the use of 8 to 33.5% hydrated kaolin clay along with a suspending agent, such as bentonite clay, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, and a dispersing agent such as a non-ionic surfactant, an anionic surfactant, and alkanolamine or the sodium salt of a polymeric carboxylic acid.
U.S. Pat. No. 4,374,203 discloses the use of anionic polymers and copolymers in combination with cationic polymers as a means of controlling calcined clay slurry viscosity and stability. The examples contained in that patent which use only an anionic copolymer do not show any advantage with regard to decreasing the dilatancy of the slurries.
Those approaches are only somewhat effective in solving the problems of calcined kaolin clay slurry stability and dilatancy. Also, since these methods involve the use of two or more compounds to achieve stable slurries, they present manufacturing difficulties and increased costs. In addition, dilatant settling may still occur, resulting in unusable slurries.
We have found ways to prepare calcined clay slurries which solve the problems mentioned above. Stable calcined clay slurries can be made containing at least 10% by weight calcined clay solids by adding to said slurries an effective amount of anionic acrylic emulsion copolymer.
It is an object of this invention to provide a method of preparing stable aqueous slurries of clay, especially calcined kaolin clay, which do not exhibit dilatant settling even after prolonged storage without agitation, and which can be pumped without suffering the effects of dilatancy.